The present invention relates to an image forming apparatus employing electrophotographic technology and particularly to an image forming apparatus which transfers a visible toner image formed on a latent image carrier to a recording medium electrostatially.
In a conventional image forming apparatus, a photoreceptor as a latent image carrier such as a photosensitive drum or a photosensitive belt is rotatably supported to the main body of the image forming apparatus. During the image forming operation, a latent image is formed onto a photosensitive layer of the photoreceptor and, after that, is developed with toner particles to form a visible image. Then, the visible image is transferred to a recording medium. For transferring the visible image, there are a method of directly transferring the visible image to the recording medium by using a corona discharge or a transferring roller, and a method of transferring the visible image to the recording medium via an intermediate transfer member such as a transfer drum or a transfer belt, that is, transferring the visible image twice.
These methods are employed in monochrome image forming apparatuses. In addition, for a color image forming apparatus having a plurality of photoreceptors and developers, there is a known method transferring a plurality of color images on a transfer belt or transfer drums to a recording medium such as a paper in such a manner that the respective color images are sequentially superposed on each other, and then fixing these images. The apparatuses according to such a method using a belt are categorized as a tandem type while the apparatuses according to such a method using drums are categorized as a transfer drum type. Moreover, an intermediate transferring type is also known in which color images are sequentially primary-transferred to an intermediate transfer medium and the primary-transferred images are secondary-transferred to a recording medium such as a paper at once. Arranged on the photoreceptor used for any of the aforementioned methods is a cleaning mechanism for cleaning toner particles after developing and residual toner particles remaining on the photoreceptor after the transferring.
As toner used for such an image forming apparatus, dual-component toner composed of a developer and a magnetic carrier is generally known. Though the dual-component toner achieves relatively stable developing, the mixing ratio of the developer and the magnetic carrier is easily varied so that the maintenance for the mixing ratio is required. Accordingly, magnetic single-component toner has been developed. However the magnetic single-component toner has such a problem that clear color images are not obtained due to the opacity of magnetic material thereof. On the other hand, non-magnetic single-component toner has been developed as color toner. For obtaining high-quality record images with the non-magnetic single-component toner, there is a problem how to uniformly charge the toner particles
In order to solve the aforementioned problem of the non-magnetic single-component toner, Japanese Patent Unexamined publication H3-62072 discloses a toner layer thickness regulating member for a developing device. The toner layer thickness regulating member is made of a metal of which work function is low so as to have not only a function controlling the thickness of a toner layer but also a function actively causing triboelectric charging, thereby making charge uniform. This avoid local variation in the developing concentration due to insufficient charge, prevents deterioration of quality of record images, and equalize the thickness of toner layer As a similar technique, Japanese Patent Unexamined Publication H3-23347 discloses a developer carrying member (development roller), a developer controlling means, and a developer which are set to satisfy a relation (Wd−Wt)×(Wb−Wt)>0, wherein Wd, Wb, and Wt are respective work functions of the developer carrying member, the developer controlling means, and the developer, thereby reducing inversely-charged toner particles and low-charged toner particles. Even when the relation of the work functions of the aforementioned three components is satisfied as disclosed in the publication, there are problems that a phenomenon called “fog”, in which non-image portions are developed, may still occur because toner particles have a particle size distribution and that it is impossible to increase the transfer efficiency.
As for color image apparatuses, the modern trend is toward the use of toner of small particle size, uniform, and high circularity in order to improve the transfer efficiency. However, the use of such a toner reduces the fluidity of toner due to the small particle size so that it is hard to cause triboelectric charging relative to a development roller or a toner layer thickness regulating member. As a result, it is impossible to give sufficient charge. In case of toner for negative charge, there is a problem that some toner particles may be positively charged due to inductive charge.
Particularly, in an image forming apparatus which forms images by negative charge reversal developing, there is a problem of the toner and a photoreceptor that positively charged toner particles on non-image portions of a latent image carrier (photoreceptor) make “fog”, thus increasing the actual consumption of toner and also increasing the cleaning load of the photoreceptor. If a large amount of superplasticizing agent is added as an external additive to the toner in order to resolve the aforementioned problem, there may be another problem of reducing the fixing property. In a color image forming apparatus using an intermediate transfer medium, there is a problem that positively charged toner particles on a photoreceptor, if any, reduce the transfer efficiency to the intermediate transfer medium.
It is a first object of the present invention to provide an image forming apparatus of a type developing a latent image on a latent image carrier (photoreceptor) with negatively charged toner particles, in which there is little fog on non-image portions of the photoreceptor during developing and it is possible to improve the transfer efficiency.
It is a second object of the present invention to provide an image forming apparatus employing a developing device of a type developing a latent image on a latent image carrier with negatively charged toner particles, in which in a process of transferring a visible image developed on the latent image carrier to an intermediate transfer medium, the charge of positively charged toner particles adhering to the latent image carrier is reduced, thereby increasing the transfer efficiency to the intermediate transfer medium.
It is a third object of the present invention to provide an image forming apparatus which can minimize the consumption of toner particles so as to reduce the amount of toner particles to be cleaned, thereby reducing the running cost and reducing the size of a cleaning container.